Overload relays of various sorts have long been utilized in connection with the operation of electrical equipment, particularly electrical equipment drawing relatively high levels of power. Various devices have been utilized to sense a current flow and to trip a switch or the like when the current level exceeds some predetermined value. Frequently, electromechanical devices employing heaters have been utilized for the purpose. Frequently also, overload circuits of this type require an external source of power to drive a current interrupting switch when an overload has been sensed.
While, in many instances, such prior devices perform their intended functions well, they are not without their undesirable features. For example, in the case of those systems requiring an external source of power, if the external power source fails for any reason, it is either impossible to trip the circuit in response to an overload or a trip occurs when no overload is, in fact, occurring. Those systems employing heaters tend to be bulky in order to house the heater and a bimetal typically associated therewith. Moreover, concern must be given in their design for a means to dissipate heat at a relatively uniform rate so as to enable them to function accurately when reset immediately following a trip.
Many examples of prior art overload relays of the type generally discussed above, also do not provide protection against the loss of a phase when the overload relays are utilized in monitoring a multi-phase load. While loss of a phase will typically result in a substantial increase in the current flowing in the remaining phases which will ultimately lead to a trip of the relay, it is desirable to trip the overload relay much more rapidly when a phase is lost so as to prevent any overheating due to excessive current flow of the operative phases within the load.
The present invention is directed to overcoming these and other problems.